The syndrome of medullary carcinoma of the thyroid, pheochromocytoma and parathyroid hyperplasia (Multiple Endocrine Neoplasm Syndrome - Type II) is transmitted genetically as an autosomal dominant with a high degree of penetrance. The frequency of medullary carcinoma in children of an affected parent is estimated to be approximately 50%. Annual screening of all family members 3 years of age and older in a large New Haven kindred has been commenced using a modification of the pentagastrin stimulation test reported by Hennessey and co-workers (J. Clin. Endocrinol. Metabl. 39:487, 1974). Family members are studied on an annual basis using the ambulatory facilities of the Adult (General) Clinical Research Center and of the Children's Clinical Research Center. A pentagastrin infusion, 0.5 mcg/kg is performed with serum for calcitonin immunoassay obtained before and at 2, 5, 10 and 20 minutes following. Radioimmunoassay of calcitonin is being performed in the laboratories of Dr. Armen H. Tashjian, Harvard School of Dental Medicine. Screening for parathyroid disease with total and ionized calcium, alkaline phosphatase and immunoreactive parathormone are obtained simultaneous with pentagastrin infusion. Urinary catecholamines are assayed in the laboratory of Dr. Lewis Landsberg, Beth Israel Hospital, Boston, to monitor for development of pheochromocytoma. The program has now been extended to two other families in the Southern Connecticut area. Of the more than 120 members of the original kindred, 88 members older than 3 years have been studied with no abnormalities detected in family members not previously recognized to have manifestations of this familial cancer syndrome.